I would like to be able to claim contracts. Doing so will make it so the contract cannot be fullfilled by anyone else.
There should also be a downside to claiming contracts however. I think it's only fair to give the player a penalty for failing to complete contracts.
This could be a fine, maybe 50% of the value of the contract.
This could also be a hefty reputation hit from the faction the contract belongs to.
Maybe lock the player out of contracts alltogether for some time.